How can you create a surface from combining 2 objects?
A tool which will cut your mesh in any case regardless of topology is Bisect.

Let's say we want to cut this:



Got to Edit Mode, Side View, and activate the Bisect Tool from the Tool Shelf:



Drag a line across your model, it will be the cutline projected from the view you're looking from through the whole model. The cut will be made, and the cutline selected:



In perspective:



Now, select one of the newly created vertices, and run the Select Side of Active Operator from the menu:



Tweak the operator settings to fit your situation:



Now, deselect the parts you want to keep, and delete the faces afterwards:



The whole process is a bit lengthy, because you have both faces you want to keep and faces you'd like to delete. If you manage to separate out the faces to cut beforehand, you can tweak the Bisect settings directly, as it has an option to remove either top or bottom half of the mesh.